1. Field of the Invention
The present invention relates to a serial axial fan, and more specifically to a serial axial fan including two impellers arranged in series.
2. Description of the Related Art
Blower fans are widely used to cool household electrical appliances, office equipment, industrial equipment, and so on, for air conditioning or ventilation, or as blowers for use in vehicles and so on. As such a blower fan, a serial axial fan including two axial fans connected in series along a central axis is known. For example, a counter-rotating axial blower is disclosed in JP-A 2004-278370. In this blower, a first impeller and a second impeller are arranged in series along a central axis inside a housing. The first and second impellers are arranged to rotate in mutually opposite directions.
JP-A 2002-21777 discloses a jet fan installed on a ceiling of a tunnel. The jet fan includes a first impeller, a second impeller, and reverse rotation means arranged to cause the first and second impellers to rotate in mutually opposite directions. Each of the first and second impellers is arranged to be reversible in rotation. JP-A 2009-250225 discloses an axial blower. The axial blower includes two impellers arranged in series, and is arranged to be capable of rotating in both a normal direction and a reverse direction. Each of rotor blades of each impeller has a curved cross-section.
The serial axial fan is typically arranged to send air in a fixed direction as is the case with the counter-rotating axial blower disclosed in JP-A 2004-278370. Therefore, each blade is arranged to have a shape appropriate for sending air in the single fixed direction. In the counter-rotating axial blower disclosed in JP-A 2004-278370, for example, each of front and rear blades has a curved shape with a concave portion thereof being open toward an outlet side.
Meanwhile, depending on a purpose of the blower fan, the blower fan is demanded to be capable of sending air in both directions equivalently, and an increase in static pressure is demanded for each of the case where the blower fan sends air in one direction and the case where the blower fan sends air in an opposite direction. In the case where blower fans having the same design are installed on a variety of devices, for example, it is desirable that the blower fans should be capable of sending air in both directions. However, a blower fan designed to send air in a single fixed direction suffers a significant decrease in a static pressure characteristic when sending air in an opposite direction.
The jet fan disclosed in JP-A 2002-21777 is capable of sending air in both directions. Each blade of the jet fan is in the shape of a flat plate, and therefore, high static pressure cannot be obtained. The axial blower disclosed in JP-A 2009-250225 is also capable of sending air in both directions. However, because the two impellers are arranged to rotate in the same direction when viewed along a central axis, an outgoing air current has a large whirl component and spreads radially. Therefore, high static pressure cannot be obtained.